Method for decreasing nitrogen oxides in hydrocarbon-containing exhaust gases using an scr catalyst based on a molecular sieve

ABSTRACT

The invention relates to a process for treating diesel engine exhaust gases comprising nitrogen oxides (NO x ) and hydrocarbons (HC) by selective catalytic reduction of the nitrogen oxides with ammonia or a compound decomposable to ammonia as a reducing agent over an SCR catalyst based on a molecular sieve. The properties of the catalyst used are such that the hydrocarbons present in the exhaust gas are kept away from the catalytically active sites in the catalyst over which the reactions take place by the molecular sieve-like action of the zeolite present in the catalyst. This prevents HC-related degradation and aging effects of the SCR catalyst and achieves a considerable improvement in nitrogen oxide conversions in HC-containing exhaust gas.

The invention relates to a process for reducing the level of nitrogen oxides in the exhaust gas of internal combustion engines operated predominantly under lean conditions. More particularly, the invention relates to a process for treating diesel engine exhaust gases comprising nitrogen oxides and hydrocarbons by selective catalytic reduction of the nitrogen oxides with ammonia or a compound decomposable to ammonia as a reducing agent over an SCR catalyst based on a molecular sieve.

In addition to the pollutant gases which result from incomplete combustion of the fuel, these being carbon monoxide (CO) and hydrocarbons (HC), the exhaust gas of diesel engines comprises particulate material (PM) and nitrogen oxides (NO_(x)). In addition, the exhaust gas of diesel engines contains up to 15% by volume of oxygen. It is known that the oxidizable pollutant gases, CO and HC, can be converted to harmless carbon dioxide (CO₂) by passing them over a suitable oxidation catalyst, and particulates can be removed by passing the exhaust gas through a suitable particulate filter. Technologies for removal of nitrogen oxides from exhaust gases in the presence of oxygen are also well known in the prior art. One of these “denoxing” processes is the SCR process (SCR=Selective Catalytic Reduction), i.e. the selective catalytic reduction of the nitrogen oxides with the reducing agent ammonia over a catalyst suitable therefor, the SCR catalyst. It is possible to add ammonia as such to the exhaust gas stream, or in the form of a precursor compound decomposable to ammonia under ambient conditions, “ambient conditions” being understood to mean the environment of the compound decomposable to ammonia in the exhaust gas stream upstream of the SCR catalyst. To perform the SCR process, a source for providing the reducing agent, an injection apparatus for metered addition of the reducing agent as required into the exhaust gas and an SCR catalyst arranged in the flow path of the exhaust gas are needed. The totality of reducing agent source, SCR catalyst and injection apparatus arranged on the inflow side to the SCR catalyst is referred to as an SCR system.

To comply with the exhaust gas limits for diesel motor vehicles which apply in the USA and in Europe, it has been sufficient to date to remove only some of the pollutants in the exhaust gas by exhaust gas aftertreatment processes. The formation of the pollutant gases remaining was reduced by appropriate calibration of the combustion conditions within the engine to such an extent that the limits could be complied with without additional exhaust gas aftertreatment. For example, by selection of appropriate calibration points in the combustion within the engine, the emission of nitrogen oxides could be kept so low that no exhaust gas aftertreatment for removal of nitrogen oxides was needed. On the other hand, the exhaust gas did contain relatively large amounts of carbon monoxide (CO), uncombusted hydrocarbons (HC) and particulates (PM), which were removable, for example, by a series connection of diesel oxidation catalyst and diesel particulate filter in the exhaust gas system. This process is still being used today, especially in automobiles with diesel engines.

FIG. 1 shows a schematic of the conflicting aims of reduction in particulates and NO_(x) by measures within the engine, and the achievability of the corresponding EU-IV/EU-V limits. A reduction in particulate emission as a result of measures within the engine results in a rise in the nitrogen oxide contents in the untreated emission, and necessitates downstream denoxing of the exhaust gas (1). Vice versa, a reduction in nitrogen oxide emissions by virtue of measures within the engine leads to an increase in particulate emission and requires the use of a diesel particulate filter (2) for attainment of the prescribed limits.

The use of diesel particulate filters in commercial vehicles is undesirable owing to the unit sizes required because of greater exhaust gas mass flows and the exhaust gas pressure drops associated with the installation thereof. Therefore, HC and particulate emissions in commercial vehicle applications have to date been reduced within the engine to such an extent that a specific exhaust gas aftertreatment is not needed to comply with the prescribed particulate limits. Instead, the nitrogen oxides emitted to an increased degree are removed by an SCR system, which may be preceded upstream by a diesel oxidation catalyst to increase the low-temperature conversion.

With the even stricter limits that will be prescribed in the future, measures within the engine will generally no longer be sufficient to reduce the level of individual pollutant gases. Exhaust gas aftertreatment to remove all pollutant gases emitted by the engine will be obligatory in general for diesel vehicles newly registered from 2010. It will thus be necessary for the present applications for diesel exhaust gas aftertreatment to combine diesel oxidation catalyst, diesel particulate filter and SCR systems, though the combination of these units entails altered operating conditions for the SCR catalyst in particular. Three systems of this kind are currently being tested: in the “SCRT® system” according to EP 1 054 722, a diesel oxidation catalyst, a diesel particulate filter and an SCR system are arranged in succession in the flow direction of the exhaust gas. Alternatively, the SCR system may be arranged between a close-coupled diesel oxidation catalyst and a diesel particulate filter in the underbody of the vehicle (DOC-SCR-DPF) or upstream of a unit composed of diesel oxidation catalyst and diesel particulate filter (SCR-DOC-DPF).

The combination of diesel particulate filter and SCR system in an exhaust gas line means that the SCR catalyst is exposed to significantly higher HC concentrations over long periods at particular operating points than has been the case in applications to date. There are several causes of these increased HC concentrations:

Firstly, the combustion within the engine is no longer calibrated with the aim of dispensing with costly exhaust gas aftertreatment stages, at one of the extreme points on the combustion map, but from the aspect of optimizing performance, with particulates and HC, and also nitrogen oxides, being tolerated equally as emissions (cf. point (3) in FIG. 1). This causes a certain basic level of HC stress on the exhaust gas aftertreatment system, the exhaust gas already having significantly higher HC concentrations than in the applications customary to date, which were calibrated for the avoidance of particulates (and HC), in which SCR systems were used. Secondly, the diesel particulate filter has to be regenerated at regular intervals, which is accomplished by controlled burnoff of the particulate load among other ways. For this purpose, the filter has to be heated to a temperature above the soot ignition temperature. This “heat-up” is effected by postinjection of fuel into the piston exhaust stroke of the cylinder or into the exhaust gas line and by catalytic conversion of the uncombusted hydrocarbons on an oxidizing catalyst (“heat-up catalyst”). Usually, an upstream diesel oxidation catalyst assumes the function of the “heat-up catalyst”. If this is not present, as in the SCR-DOC-DPF system, it is also possible—according to the catalyst formulation—for the SCR catalyst to assume “heat-up” functions. In each case, higher HC concentrations are present during the filter regeneration upstream of the SCR catalyst, since the hydrocarbons injected after ignition are not fully combusted catalytically during the “heat-up”. In an SCRT® system, in which diesel oxidation catalyst and diesel particulate filter are upstream of the SCR catalyst, there is additionally prolonged HC stress on the SCR catalyst after a certain run time, which is attributable to the hydrothermal aging of the oxidation functions in the diesel oxidation catalyst and in the optionally catalytically coated filter.

Irrespective of a regeneration of the diesel particulate filter, further heating measures by postinjection of fuel may be necessary to compensate for cold start delays, and lead to transient sharp increases in the HC concentration upstream of the SCR catalyst.

The effects mentioned have the result that the SCR catalyst in modern combined emission control systems is exposed to altered operating conditions, the HC contents present in the exhaust gas upstream of the SCR catalyst being much higher than in applications to date. Under these conditions, conventional SCR catalysts generally exhibit a clear decline in nitrogen oxide conversion performances.

For example, the “conventional” zeolite catalysts described in U.S. Pat. No. 4,961,917 store significant amounts of HC in the zeolite pores. They exhibit satisfactory nitrogen oxide conversion rates only when the hydrocarbon emissions have been removed virtually completely before entry into the SCR catalyst, for example by means of a suitable upstream oxidation catalyst.

EP 0 385 164 B1 describes unsupported catalysts for selective reduction of nitrogen oxides with ammonia, which comprise, in addition to titanium oxide and at least one oxide of tungsten, silicon, boron, aluminum, phosphorus, zirconium, barium, yttrium, lanthanum and cerium, an additional component selected from the group of the oxides of vanadium, niobium, molybdenum, iron and copper. Some of these catalysts exhibit significant oxidation activity toward hydrocarbons. As a result, there is exothermic combustion of the hydrocarbons present in the exhaust gas over the SCR catalyst, which can lead to premature thermal damage to the SCR functionality in the case of high amounts of HC.

It was an object of the present invention to specify an apparatus for reducing the NO_(x) content of a hydrocarbon-comprising stream of exhaust gases of an internal combustion engine operated under lean conditions by means of the ammonia-operated SCR system, which is configured in an improved manner compared to the prior art. More particularly, the NO_(x) reduction performance in hydrocarbon-comprising diesel engine exhaust gases should be improved by use of an SCR catalyst advantageous for such applications.

The object is achieved by a process for treating diesel engine exhaust gases comprising nitrogen oxides (NO_(x)) and hydrocarbons (HC), comprising: a) the addition of ammonia (NH₃) as such or in the form of a compound which gives rise to ammonia under ambient conditions from a source which does not form part of the exhaust gas line to the exhaust gas stream comprising nitrogen oxides and hydrocarbons; and b) the selective reaction of NO_(x) with the NH₃ added to the exhaust gas stream over an SCR catalyst comprising a zeolite exchanged with copper (Cu) and/or iron (Fe). To achieve the object, the properties of the zeolite present in the catalyst must be such that the hydrocarbons present in the exhaust gas are kept away from the active sites in the catalyst over which the reactions take place by the molecular sieve-like action of the zeolite.

Studies of the light-off performance of common SCR catalysts show that nitrogen oxide reduction with ammonia generally sets in only when hydrocarbons present in the exhaust gas have been converted completely. This indicates that the hydrocarbons reversibly block the catalytically active sites required for the reduction of the nitrogen oxides with ammonia in transient metal-based, and also in conventional zeolite-based, SCR catalysts, thus at least delaying the comproportionation of the nitrogen oxides with ammonia, or even preventing it depending on the operating temperature.

It has now been found that, surprisingly, this delaying or prevention of the comproportionation of nitrogen oxides with ammonia to nitrogen is not observed in the presence of hydrocarbons when the reaction is effected over an SCR catalyst based on a Cu- and/or Fe-exchanged zeolite, said zeolite having a maximum lower channel width of 2.6 Å-4.2 Å. By passing the diesel engine exhaust gas comprising nitrogen oxides and hydrocarbons over such an SCR catalyst after addition of ammonia as such or of a precursor compound decomposable to ammonia from a source which does not form part of the exhaust gas line, it is astonishingly possible to selectively react NO_(x) with the NH₃ added to the exhaust gas stream without influencing the SCR activity by the presence of other molecules, for example hydrocarbons. The molecular sieve-like action of the Cu- and/or Fe-exchanged zeolite used keeps such hydrocarbons away from the active sites over which the reactions take place. This causes a significant rise in (long-term) activity of the SCR catalysts used in the process according to the invention.

Zeolites may have differently structured channels in one and the same material. The channel widths for the particular orifices of the channels may therefore have different lower and upper channel widths (definition: crystallographic free diameters of the channels in Å) (Ch. Baerlocher, Atlas of Zeolite Framework Types, 5th revised edition, 2001, ISBN: 0-444-50701-9). In these cases, several larger lower channel widths arise for the particular kinds of channels. The same applies when mixtures of such materials find use. In these cases, the larger lower channel widths are based on the material with the smaller channel widths in each case. It is thus sufficient when at least one channel width of the zeolite used has a channel width within the specified range of 2.6 Å- 4.2 Å.

When catalysts (based on their presence in the wash coat as a catalytically active component) based on mixtures of zeolites are used, this means that inventive embodiments predominate when at least 40% by weight of the catalyst consists of Cu- and/or Fe-exchanged zeolites in which a maximum lower channel width of 2.6 Å-4.2 Å exists. Further preferably, the catalyst contains at least 50% by weight, more preferably at least 60% by weight, especially preferably at least 70% by weight and most preferably at least 80% by weight of such materials.

Within the limits specified, the person skilled in the art is free to select the larger lower channel widths of the zeolites used. What is important is that the maximum lower channel width of the zeolite used is selected such that ammonia and nitrogen oxides still find access to the active sites within the zeolite, but hydrocarbons are as far as possible prevented from diffusing into the channels. In the case of zeolites with a plurality of kinds of channels, the maximum lower channel width is crucial here.

Zeolites commonly refer to crystalline aluminosilicates with a porous framework structure composed of corner-linked AlO₄ and SiO₄ tetrahedra (W. M. Meier, Pure & Appl. Chem. 58, 1986, 1323-1328). By their nature, these have a negative excess charge in the lattice, which must be balanced by intercalation of positive ions (e.g. H⁺, Na⁺, NH₄ ⁺). The ions can be selected freely. In the present case, some Fe or Cu ions are selected as counterions (see above). How many ions can be intercalated is also guided by the ratio of aluminum to silicon atoms in the crystal lattice. For the present invention, it is advantageous when the molar ratio of SiO₂ to Al₂O₃ in the zeolite is in the range from 5 to 100. A range from 10 to 60 is preferred, and from 15 to 45 is very particularly preferred.

Zeolites which satisfy the requirements specified are familiar to those skilled in the art. Zeolites usable in accordance with the invention can be found, for example, in the literature Ch. Baerlocher, Atlas of Zeolite Framework Types, 5th revised edition, 2001, ISBN: 0-444-50701-9. The materials in the present case are Fe- and/or Cu-exchanged zeolites. These and preparation thereof are described especially in the literature (K. Sugawara, Appl. Catal. B. 69, 2007, 154-163; W. Arous et al., Top. Catal. 42-43, 2007, 51-54; Ishihare et al., J. Catal. 169, 1997, 93-102). Preference is given to zeolites selected from the group consisting of ferrierite, chabazite and erionite. Very particular preference is given to the use of ferrierite.

In the context of the invention, “catalyst based on a Cu- and/or Fe-exchanged zeolite” means that the zeolite has Cu and/or Fe in place of the positive counterions originally present. The person skilled in the art can adjust the content of Fe and/or Cu ions in the zeolite according to his or her specialist knowledge. An advantageous value is 0.1-10% by weight of the ions based on the weight of the zeolite. The ratio is preferably 1-8% by weight and most preferably 1.5-6% by weight.

The use of the zeolites ferrierite, chabazite and erionite as constituents of SCR catalysts is already known in the prior art. Like zeolite beta, zeolite Y, zeolite A or mordenite, ferrierite, chabazite and erionite exhibit a good activity in the selective catalytic reduction of nitrogen oxides with ammonia in HC-free or at least in low-HC diesel engine exhaust gas, “low-HC” exhaust gas referring to one which has an HC content of not more than 30 ppm. However, as soon as the hydrocarbon content in the exhaust gas attains or exceeds a lower threshold of 50 ppm, zeolites with a larger lower channel width of more than 4.2 Å exhibit an ever greater decline in activity in the comproportionation of NO_(x) with NH₃ with increasing catalyst load. This decline in activity is particularly marked for SCR catalysts based on zeolite beta, zeolite Y, zeolite A or mordenite. Copper (Cu)- and/or iron-exchanged zeolites of the ferrierite, chabazite and erionite structure type surprisingly do not exhibit this decline in activity in hydrocarbon-containing diesel engine exhaust gases. In contrast to the aforementioned zeolites beta, A, Y and MOR, their properties are such that the hydrocarbons present in the exhaust gas, owing to the molecular sieve-like action of the zeolites, are kept away from the catalytically active sites in the catalyst over which the reactions take place. In this way, the reversible blockage of the catalytically active sites is prevented, and aging phenomena which can arise as a result of thermal regeneration effects on the zeolite and which significantly reduce the long-term stability of conventional SCR catalysts in HC-containing diesel engine exhaust gases are avoided.

The advantages of the process according to the invention become particularly clear when the hydrocarbon content in the exhaust gas before the reaction of NO_(x) with the ammonia NH₃ added to the exhaust gas stream over the SCR catalyst is at least 50 ppm. The advantage of the invention is very particularly clear in the case of hydrocarbon contents in the diesel engine exhaust gas upstream of the SCR catalyst of at least 100 ppm. This is especially true when the hydrocarbon content in the exhaust gas before the reaction of NO_(x) with NH₃ over the SCR catalyst (in step b)) has transient peak values of at least 300 ppm or 500 ppm. It is pointed out that, according to the application and mode of operation of the vehicle, it is by no means rare for HC peak values upstream of the SCR catalyst of 1000 ppm or more to be observed. In heavy goods vehicle applications and in diesel engine-operated machinery, HC peaks of up to 2% by volume are observed in the exhaust gas upstream of the SCR catalyst. In such cases, use of HC-resistant SCR catalysts, as form the basis of the process according to the invention, is essential since the “conventional” SCR catalysts based on zeolite beta, A, Y or MOR become saturated with hydrocarbons under these conditions and no longer exhibit any NO_(x) conversion with ammonia whatsoever.

The inventive catalysts comprising Cu- and/or Fe-exchanged zeolites, the properties of which are such that they keep the hydrocarbons present in the exhaust gas away from the active sites in the catalyst over which the reactions take place by virtue of their molecular sieve-like action, preferably do not make any crucial contribution to the reduction in the hydrocarbon content in the exhaust gas under regular running cycle conditions. In that case, the process according to the invention is characterized in that the hydrocarbon content in the exhaust gas is not significantly reduced after the reaction of NO_(x) over the SCR catalyst in step b) at an exhaust gas temperature at the inlet of the SCR catalyst of up to 180° C., when said SCR catalyst comprises a zeolite exchanged with copper (Cu). When the inventive SCR catalyst comprises iron instead of copper as the exchange ion, the hydrocarbon content in the exhaust gas is not significantly reduced after the reaction of NO_(x) over the SCR catalyst in step b) at an exhaust gas temperature at the inlet of the SCR catalyst of up to 270° C.

The process according to the invention can be performed in an apparatus in which an oxidation catalyst and/or an optionally coated particulate filter are arranged upstream of the SCR system, i.e. upstream of the inventive SCR catalyst and the corresponding injection device for a reducing agent (NH₃ or precursor compound). Oxidation catalysts suitable for this purpose can be found in the literature (EP 1 255 918 B1, US 20050201916). Suitable particulate filters can likewise be found in the literature (EP 1 250 952 A1, WO 2006/021337 A1). The apparatus for performing the process according to the invention may also comprise, downstream of the SCR catalyst, an oxidation catalyst which helps to prevent any NH₃ slippage present (WO 2007/004774 A1). Also conceivable are exhaust gas apparatuses in which an SCR system is arranged as described in the exhaust gas line upstream of an oxidation catalyst and/or an optionally coated particulate filter. Likewise conceivable is the arrangement in which the inventive SCR system with a catalyst based on a Cu- and/or Fe-exchanged zeolite is arranged between the oxidation catalyst and optionally coated particulate filter.

The injection apparatuses used can be selected as desired by the person skilled in the art. Suitable systems can be found in the literature (T. Mayer, Feststoff-SCR-System auf Basis von Ammoniumcarbamat [Solid-state SCR system based on ammonium carbamate], Thesis, Technical University of Kaiserslautern, 2005). The ammonia can be introduced into the exhaust gas stream via the injection apparatus as such or in the form of a compound which gives rise to ammonia under the ambient conditions. Useful compounds of this kind include aqueous solutions of urea or ammonium formate, and likewise solid ammonium carbamate. These can be drawn from a source provided, which is known per se to the person skilled in the art (FIG. 2), and supplied to the exhaust gas stream in a suitable manner. The person skilled in the art more preferably uses injection nozzles (EP 0311758 A1). By means thereof, the optimal ratio of NH₃/NO_(x) is established, in order that the nitrogen oxides can be converted very substantially to N₂.

The exhaust gas originating from the combustion operation, for example once it has passed over an optional oxidation catalyst and/or an optionally coated particulate filter, is supplied with ammonia or the precursor compound in appropriate amounts via the injection apparatus. Subsequently, it is passed over the SCR catalyst. The temperature over the SCR catalyst should be between 150° C. and 500° C., preferably between 200° C. and 400° C. or between 180° C. and 380° C., in order that the reduction can take place as completely as possible. Particular preference is given to a temperature range from 225° C. to 350° C. for the reduction. In addition, optimal nitrogen oxide conversions are achieved only when a molar ratio of nitrogen monoxide to nitrogen dioxide is present (NO/NO₂=1) or the NO₂/NO_(x) ratio=0.5 (G. Tuenter et al., Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 633-636). Optimal conversions beginning with 75% conversion at only 150° C. with the same optimal selectivity for nitrogen are achieved according to the stoichiometry of the reaction equation

2 NH₃+NO+NO₂>2 N₂+3 H₂O

only with an NO₂/NO_(x) ratio of 0.5. This applies not only to SCR catalysts based on iron-exchanged zeolites but to all common, i.e. commercially available, SCR catalysts.

Metal-exchanged beta-zeolites (maximum lower channel width 6.6 Å) as catalyst materials exhibit a high activity for the reduction of NO_(x) by means of ammonia in what are known as SCR systems. In the presence of hydrocarbons in the exhaust gas, they exhibit, as described, however, a severe decline in activity (example 3, FIG. 4 vs. FIG. 3). According to the inventors' findings, this is attributable to a poisoning or storage effect, in which the intercalated hydrocarbons occupy the active sites of the catalyst and thus prevent the desired comproportionation reaction. The process is reversible in principle, but every regeneration of the beta-zeolite-based catalyst causes a degradation of the SCR activity owing to thermally induced aging processes.

For further illustration of the advantageous use of metal-exchanged zeolites with a maximum lower channel width of 4.2 Å as the SCR catalyst, the test described below, consisting of five phases, was carried out on a model gas system (example 4).

The example used is an SCR catalyst based on Fe-ferrierite (3.5% Fe), and the comparative example an Fe-beta-zeolite (referred to as Fe-beta—likewise 3.5% Fe). To illustrate the test procedure, the metering and temperature profile of the test is shown in FIG. 5. The profiles of the NO_(x) conversions of the two catalysts discussed below are shown in FIG. 6, and the corresponding profiles of HC and CO concentrations in the exhaust gas in FIGS. 7 and 8. It should be pointed out that the wash coat loadings of the two SCR catalyst specimens are different (165 g/l for Fe-ferrierite vs. 229 g/l for beta-zeolite).

Phase I: Determination of NO_(x) conversion without hydrocarbons (T=300° C., 250 ppm NO, 250 ppm NO₂, 500 ppm NH₃, 5% by vol. O₂, 1.3% by vol. H₂O, balance N₂, space velocity 50 000 h⁻¹). An NO_(x) conversion of 98% is achieved for Fe-ferrierite, and 99% for Fe-beta.

Phase II: Duration 1800 s. A mixture of toluene and dodecane (1:1 w/w) containing 1000 ppm of Cl is added to the feed gas from phase I. There is barely any effect on the NO_(x) conversion of the Fe-ferrierite (94%), whereas the conversion for Fe-beta declines significantly to 55%. While the amount of HC in the exhaust gas rapidly breaks through the feed value of 1000 ppm for Fe-ferrierite, Fe-beta stores hydrocarbons over the entire duration of phase II.

Phase III: Duration 1800 s. The toluene/dodecane mixture is removed again from the feed gas; the test conditions are thus the same as in phase I. The NO_(x) conversion of Fe-ferrierite remains stable at a high level of 95%, whereas the conversion for Fe-beta recovers gradually to only 85% over the course of this phase.

Phase IV: Duration 200 s. Regeneration of the catalyst at 500° C. Whereas only small amounts of thermally desorbed hydrocarbons and CO (product of the burnoff of dodecane and toluene) are detected in the exhaust gas for Fe-ferrierite, it is found for Fe-beta that the hydrocarbons stored during phase II now desorb or burn off.

Phase V: Same conditions as in phase I. For Fe-ferrierite, the NO_(x) conversion remains stable at 95%; for Fe-beta, a value of 93% is determined after the regeneration in phase IV.

In addition to the decline in SCR activity in the presence of hydrocarbons, a further undesired effect which occurs in the case of wide-pore zeolites, particularly in the fresh state, is significant exothermicity when the hydrocarbons stored on the catalyst ignite and burn off (see example 5). As a result of the temperature rise that this causes, the catalyst undergoes aging, which leads to a reduction in the activity thereof. This is not the case for zeolites used in accordance with the invention, for example ferrierite, even in the fresh state. Since only a small amount of hydrocarbons at most is stored on the catalyst, only a small temperature rise on the catalyst occurs when they burn off. The damage to the catalyst is thus also reduced. When, for example, a Cu-beta-zeolite (Cu-beta) with a maximum lower channel width of 6.6 Å and a Cu-ferrierite with a maximum lower channel width of 4.2 Å are contacted with hydrocarbons, and the hydrocarbons are ignited by increasing the reactor temperature from 100° C. to 400° C. in an oxygenous atmosphere, a temperature peak of more than 700° C. in the exhaust gas occurs in the case of Cu-beta-zeolite owing to the burnoff of a large stored amount of intercalated hydrocarbons, whereas the exhaust gas temperature for Cu-ferrierite follows the reactor temperature without a temperature peak (FIG. 9).

DESCRIPTION OF THE FIGURES

FIG. 1: The schematic diagram of the connection between particulate emission and NO_(x) emission in the untreated exhaust gas of an internal combustion engine operated under predominantly lean conditions and the limits valid according to EU-IV/V;

-   -   (1): Reduction in untreated particulate emission down to below         the given limit by measures within the engine>reduction in         NO_(x) emission by exhaust gas aftertreatment (denoxing);     -   (2): Reduction in untreated NO_(x) emission down to below the         given limit by measures within the engine>reduction in         particulate emission (diesel particulate filter);     -   (3): Calibration of combustion within the engine according to         the aspect of optimizing performance>reduction in particulate         and NO_(x) emission by exhaust gas aftertreatment measures to         achieve the given limits.

FIG. 2: The schematic diagram of a preferred embodiment of the inventive apparatus, comprising (1) the injection apparatus for addition of ammonia or of an ammonia-generating precursor compound to the exhaust gas stream (flow direction indicated by “>”) from a source (2) which does not form part of the exhaust gas line, an SCR catalyst (3) which effectively catalyzes the comproportionation of the nitrogen oxides with ammonia within a temperature range between 150° C. and 500° C., and an optional oxidation catalyst (4) which helps to prevent any NH₃ slippage present as a result of oxidation of the NH₃ to nitrogen and water.

FIG. 3: SCR activity of Cu-ferrierite (maximum lower channel width 4.2 Å) under 500 ppm of NO, 450 ppm of NH₃, 5% O₂, 1.3% H₂O and nitrogen (blue) without and (pink) with 200 ppm of propene and 200 ppm of CO.

FIG. 4: SCR activity of Cu-beta SCR catalyst (maximum lower channel width 6.6 Å) under 500 ppm of NO, 450 ppm of NH₃, 5% O₂, 1.3% H₂O and nitrogen (blue) without and (pink) with 200 ppm of propene and 200 ppm of CO (comparative example).

FIG. 5: Temperature profile of the model gas test from example 4, divided into five phases, and metered concentrations of the NO, NO₂, NH₃ and hydrocarbon components.

FIG. 6: Profile of the NO_(x) conversion of the test from example 4 for an Fe-ferrierite and Fe-beta SCR catalyst according to example 2.

FIG. 7: Profile of the HC concentrations in the exhaust gas of the test from example 4 for an Fe-ferrierite and Fe-beta SCR catalyst according to example 2.

FIG. 8: Profile of the CO concentrations in the exhaust gas of the test from example 4 for an Fe-ferrierite and Fe-beta SCR catalyst (comparative example) according to example 2.

FIG. 9: Exhaust gas temperature downstream of a Cu-beta SCR catalyst (comparative example) and a Cu-ferrierite SCR catalyst after ignition of hydrocarbons stored thereon by increasing the reaction temperature according to example 5.

EXAMPLES Example 1 General Preparation of the Catalyst

A zeolite with a maximum lower channel width of not more than 4.2 Å is impregnated in a Lödige with copper and/or iron. After drying, the powder is calcined at 500° C. for 2 hours. The powder or a mixture of different powders of this kind is slurried in water and a binder is added (10% by weight of SiO₂ sol, commercially available). Thereafter, the wash coat obtained is used to coat a monolithic catalyst substrate which is calcined at 500° C. for 2 hours. Drill cores are taken from the monolith for model gas tests. This procedure was used to prepare the catalysts for example 3 in FIG. 3 (ferrierite with 5% Cu) and the comparative example in FIG. 4 (beta with 5% Cu).

Example 2 General Preparation of the Catalyst

A mixture of silica- and alumina-based binders (SiO₂ sol, commercially available; boehmite, commercially available) is initially charged in water. The parent zeolite of the SCR catalyst having a maximum lower channel width of not more than 4.2 Å is slurried therein. Thereafter, an amount of a suitable iron and/or copper salt corresponding to the desired metal content is added to the suspension. After grinding, the wash coat thus obtained is used to coat a monolithic substrate, and the coated substrate is calcined. Drill cores are taken from the monolith for model gas tests. This method was used to prepare the Fe-ferrierites shown in FIGS. 5 to 8 and to prepare the corresponding comparative example (Fe-beta, likewise FIGS. 5 to 8).

Example 3 Determination of SCR Activity With and Without Hydrocarbon (FIGS. 3 and 4)

The drill core to be studied, produced according to example 1, after hydrothermal aging (16 hours at 750° C., 10% O₂, 10% H₂O, balance N₂, space velocity 2200 h⁻¹), was studied in a model gas test. For this purpose, in a descending temperature sequence within the temperature range from 150° C. to 500° C., the NO conversion was determined under steady-state conditions under 500 ppm of NO, 450 ppm of NH₃, 1.3% by vol. of H₂O, 5% by vol. of O₂, balance N₂, space velocity 30 000 h⁻¹. This test was performed once in the presence of 200 ppm of propene and 200 ppm of CO in feed gas, and once in the absence of these substances.

Example 4 Determination of SCR Activity With and Without Hydrocarbon (FIGS. 5 to 8)

A drill core produced according to example 2, after hydrothermal aging (48 hours at 650° C., 10% O₂, 10% H₂O, balance N₂, space velocity 2200 h⁻¹), was studied in a model gas test consisting of five phases.

-   -   Phase I: Determination of NO conversion without hydrocarbons         (T=300° C., 250 ppm of NO, 250 ppm of NO₂, 500 ppm of NH₃, 5% by         vol. of O₂, 1.3% by vol. of H₂O, balance N₂, space velocity 50         000 h⁻¹).     -   Phase II: Duration 1800 s. A mixture of toluene and dodecane         (1:1 w/w) with 1000 ppm of Cl is added to the feed gas from         phase I.     -   Phase III: Duration 1800 s. The toluene/dodecane mixture is         removed again from the feed gas; the test conditions are thus         now the same as in phase I again.     -   Phase IV: Duration 200 s. Regeneration of the catalyst by         heating the reactor at 500° C. and cooling again to 300° C.     -   Phase V: Same conditions as in phase I.

Example 5 Determination of Exothermicity Resulting from Stored Hydrocarbons (FIG. 9)

A drill core, produced according to example 1, of the catalyst to be studied (diameter 1 inch, length 3 inches) is contacted with hydrocarbons on an engine test bed at 100° C. for a period of 60 minutes. Thereafter, the drill core is preconditioned in a model gas system at reactor temperature 100° C. for 10 minutes (10% O₂, 10% CO₂, 5% H₂O, balance N₂, total flow rate 4 m³/h), then the reactor temperature is raised to 400° C. with the same gas mixture within 30 seconds. The temperature of the exhaust gas 3 inches beyond the drill core is evaluated as a measure of the exothermicity which arises. 

1. A process for treating diesel engine exhaust gases comprising nitrogen oxides (NO_(x)) and hydrocarbons (HC), comprising: a) the addition of ammonia (NH₃) as such or in the form of a compound which gives rise to ammonia under ambient conditions from a source which does not form part of the exhaust gas line to the exhaust gas stream comprising nitrogen oxides and hydrocarbons; and b) the selective reaction of NO_(x) with the NH₃ added to the exhaust gas stream over an SCR catalyst comprising a zeolite exchanged with copper (Cu) and/or iron (Fe), characterized in that the hydrocarbons present in the exhaust gas are kept away from the active sites in the catalyst over which the reactions take place by the molecular sieve-like action of the zeolite.
 2. The process as claimed in claim 1, wherein the zeolite present in the SCR catalyst is selected from the group consisting of ferrierite, chabazite and erionite.
 3. The process as claimed in claim 2, wherein the zeolite is ferrierite.
 4. The process as claimed in claim 1, wherein the hydrocarbon content in the exhaust gas before the reaction of NO_(x) over the SCR catalyst in step b) is at least 50 ppm.
 5. The process as claimed in claim 4, wherein the hydrocarbon content in the exhaust gas before the reaction of NO_(x) over the SCR catalyst in step b) has transient peak values of at least 300 ppm.
 6. The process as claimed in claim 1, wherein the hydrocarbon content in the exhaust gas is not significantly reduced after the reaction of NO_(x) over the SCR catalyst in step b) at an exhaust gas temperature at the inlet of the SCR catalyst of up to 180° C., said SCR catalyst comprising a zeolite exchanged with copper (Cu).
 7. The process as claimed in claim 1, wherein the hydrocarbon content in the exhaust gas is not significantly reduced after the reaction of NO_(x) over the SCR catalyst in step b) at an exhaust gas temperature at the inlet of the SCR catalyst of up to 280° C., said SCR catalyst comprising a zeolite exchanged with iron (Fe). 